LU-13486 llite: restore ll_dcompare()

LU-13486 llite: restore ll_dcompare()

Revert "LU-9868 llite: Get rid of ll_dcompare" as it is causing
failures in conf-sanity test_32* due to errors with sub-mounted
ext2 filesystem dentries being invalidated.

This reverts commit 787231f53ab63c72634250f8fe9d27bc66cc4e46.

Change-Id: I5cff42df30cf91523c1dbb8ee5b8eefa1ba3a5ad
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/38498
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>